1. Field of the Invention
The present invention relates to a thin-film-type magnetic head device suitable for use in recording and reproduction of high-frequency signals.
2. Description of the Related Art
FIG. 1 shows a typical known thin-film-type magnetic head device. This known magnetic head device has a substrate 6, a lower magnetic layer 2 carried by the substrate 6, and an upper magnetic layer 1 superposed on the lower magnetic layer 2 through the intermediary of an insulating layer 7a. The clearance between the upper and lower magnetic layers 1 and 2 is reduced to form a small magnetic gap 10 at the leading side (right upper side as viewed in FIG. 1) as viewed in the direction of movement of a magnetic tape. As shown in FIG. 2 which is a top plan view of the magnetic head device of FIG. 1, a through-hole (referred to as "rear core through-hole" hereinafter) 9 is formed in the trailing side of the insulating layer 7a such that the lower magnetic layer 2 and the upper magnetic layer 1 are made to abut each other through the rear core through-hole 9. In consequence, a closed loop of the magnetic path is formed to include the upper magnetic layer 1, the abutting surfaces 4 of the upper and lower magnetic layers 1 and 2 at the region of the rear core through-hole 9, the lower magnetic layer 2 and the magnetic gap 10. In this known thin-film-type magnetic head, a membrane coil 3 is wound such that a portion of this coil penetrates the insulating layer 7a.
In order to improve the magnetic efficiency in this known thin-film-type magnetic head, it has been proposed to reduce the magnetic resistance in the region where the rear core through-hole 9 exists, as disclosed in, for example, Japanese Patent Unexamined Publication Nos. 60-45913 and 60-45914.
In recording operation of the magnetic head device, a recording electric current is supplied to the thin-film coil 3 so that a magnetic flux is generated to flow through the upper magnetic layer 1 and the lower magnetic layer 2 passing through a recording medium thereby recording signals corresponding to the recording current. In reproducing the recorded information, magnetic flux is formed through the upper and lower magnetic layers 1 and 2 as the tape runs in sliding contact with the lower magnetic layer 2 in the region where the magnetic gap 10 exists. The greater the amount of the the magnetic flux, the higher the recording and reproduction efficiencies. A greater amount of magnetic flux can be obtained by increasing the thicknesses of the upper and lower magnetic layers 1 and 2 so as to increase the cross-sectional area of the head perpendicular to the direction of the magnetic flux thereby decreasing the magnetic resistance in the path of magnetism.
Generally, however, the upper magnetic layer 1, lower magnetic layer 2 and the insulating layer 7a are formed by, for example, sputtering. Therefore, the portion of the upper magnetic layer above the insulating layer 7a and the lower magnetic layer 2 can be obtained in flat forms of predetermined thicknesses without substantial difficulty, but the thickness of the upper magnetic layer 1 is undesirably reduced at the region in which the rear core through-hole exists where the surface is tapered. In addition, the upper magnetic layer 1 is depressed in the region where the rear core through-hole 9 exists, so that the magnetic flux is concentrated to the portions of the upper magnetic layer 1 constituting the wall of the through-hole 9 where the thickness of the upper magnetic layer is small. This portion of the upper magnetic layer having the reduced thickness poses a greater magnetic resistance than other portions, thus decreasing the magnetic efficiency of the thin-film-type magnetic head.
In the thin-film-type magnetic head device disclosed in Japanese Patent Unexamined Publication No. 60-45913, the rear core through-holes are provided in plural, whereas, in the thin-film-type magnetic head device disclosed in Japanese Patent Unexamined Publication No. 60-45914, the rear core through-hole has a profile composed of convexities and concavities. These measures are intended for increasing the peripheral length of the rear core through-hole so as to increase the cross-sectional area of the upper magnetic layer in the region where the upper magnetic material exists, thereby to reduce the magnetic resistance in this region.
In these known thin-film-type magnetic head devices, no substantial problem is caused in regard to the magnetic efficiency when the frequency of the signal to be recorded or reproduced is low, unless the thicknesses of the magnetic layers constituting the path of magnetism are reduced. However, when the signal frequency is increased, a serious reduction is caused in the efficiency due to loss of energy attributable to generation of eddy currents.
A discussion will be made as to the loss of energy due to generation of eddy currents. Attention is drawn to the upper flat portion of the upper magnetic layer 1 in which the magnetic flux runs in parallel to the surface of the upper magnetic layer. The magnetic flux is allowed to flow only through a core region which is spaced by a depth .delta. from both surfaces of the upper magnetic layer 1 having a thickness t as shown in FIG. 3A, due to generation of eddy currents. The depth .delta. is represented by the following formula: ##EQU1## where .rho. represents the specific resistivity of the magnetic layers 1, 2, f represents the signal frequency and .mu. represents the magnetic permeability of the magnetic layers 1, 2.
The loss due to generation of eddy currents, therefore, can be eliminated by adopting a laminated magnetic layer in which a magnetic layer having a thickness of 2.delta. and an insulating layer are superposed alternatingly to form a laminated structure.
A problem still remains unsolved even when the upper and lower magnetic layers are constructed as laminated layers. Namely, in the region where the rear core through-hole 9 exists, the magnetic flux flows in the thicknesswise direction of the magnetic films 1 and 2 so that eddy currents are generated in the film surfaces of the magnetic layers 1 and 2 so as to restrict the path of the magnetic flux to the area under the depth of .delta. given by the formula (1) from the wall surface of the rear core through-hole 9 as shown in FIG. 3B. Thus, the magnetic flux is allowed to pass only through the regions corresponding to the hatched area 8 in FIGS. 1 and 2.
Thus, the known thin-film-type magnetic head device cannot avoid energy loss due to generation of eddy currents in the region where the rear core through-hole exists, with the result that the recording and reproduction performance is lowered particularly when the signal frequency is high.